You wrote, "However, S&S2 isn't very faithful in presenting the Marianne/Brandon relationship, either, because it does not show Marianne realizing that Willoughby was no good for her..."

On the contrary, during the scene between Elinor and Marianne on the hill, when Marianne points out where she fell and first met Willoughby, the words that Elinor speaks in the novel are actually put in Marianne's mouth - something to the effect of But does it follow that if he had married me, he would have been happy? He would have had a wife he loved, but no money. He would soon begin to place the demands of his pocketbook above the demands of his heart (forgive me if I don't have it exactly verbatim from the film). IMO, this does an excellent job of showing that Marianne has come to see that Willoughby would not have been a good husband or partner for her.

Now, I don't know if the original screenplay has Elinor saying those words, and they ended up being spoken by Marianne in later iterations, but as the film stands, Marianne speaks contritely and resignedly about Willoughby's shortcomings.